In the past ten to fifteen years there has been considerable evolution in the pickling of steel. The art has progressed from simple dip tanks to horizontal pickling tanks and on to the extremely advanced turboflow systems, which led to the generation of more ecological and economical processes, all the while improving the quality of the treatment that the material had to undergo. Nevertheless, virtually all of the newer treatment facilities (primarily steel strip picking plants) were designed for hot strips with thicknesses ranging from 1.8 to 6 millimeters. Average strip thickness throughout the world is about 3 millimeters.
Today, thanks to the hot rolling technology combined with the thin slab casting technology, the hot strip sector produces thicknesses as low as 0.7 millimeters while retaining the ability to handle 6 mm in thickness. In order to produce substantially thinner hot strips in the plants, the strip must run at higher speeds during treatment. For example, with a production of 1.5 million tons per year and a 3 mm thick reference strip, strip speed is 250 meters per minute. With a 1.5 mm thick strip, processing speed at the same production capacity is 500 meters per minute; 400 meters per minute is to be expected for strip 2.4 mm thick at the same production rate.
Pickling technologies currently available on the market are generally not compatible with such high speeds, since the facility is usually designed to propel the strip through a horizontal pickling plant under low tension. This presents guiding problems, and the added condition of high speed of the strip causes the strip to carry the pickling liquid on its surface. If the strip is propelled horizontally into the acid bath with considerable kinetics, the quantity of liquid carried away may be so great that adequate pickling and safe operation are difficult to guarantee. A boundary layer of acid tends to remain stationary with respect to the strip, resulting in poor contact efficiency.
In the past, pickling tanks and their covers have been constructed to control acid vapors, as in U.S. Pat. Nos. 3,803,996 and 3,648,593 to Marshall and 4,592,784 to Ghizzi. Weirs have been used to create cascades of acid from one tank or zone to the next. See Hampel U.S. Pat. No. 3,473,962 and Matiussi U.S. Pat. No. 5,179,967. Acid has been collected in separate vessels for recycling, as in Galloway U.S. Pat. No. 4,007,750 and Gravert et al U.S. Pat. No. 5,853,495; note also Zednicek et al U.S. Pat. No. 5,716,455, which discloses restrictions constructed to shear the acid on the strip, causing turbulence; the acid is recycled through drains. In pickling wire, Hone et al in U.S. Pat. Nos. 4,950,333 and 4,951,694 utilize the dynamics of the process by generating waves of acid, which are controlled by weirs.
Acid is recycled from a high end of an acid tank to a low end by gravity through a pipe, as configured by Lordo in U.S. Pat. No. 5,803,981. Kimura et al, in U.S. Pat. No. 5,116,447, recycles "wakes" of acid stripped by weir members shaped to direct the excess acid to the sides of the weirs for draining.
As indicated above, the kinetics of the extremely high speed of the newer pickling lines results in rapid movement of the acid in the bath, particularly that above the strip, towards the downstream end of the process and apparatus. This causes increasing depths of acid in the downstream ends of the pickling vessels, compounding the hazards for workers, and causing environmental problems due to the possibility of acid escaping from the apparatus, and economic loss from the underuse of the acid.